g(g(x))=16x-15
means that when you subsituted g(x) for x in g(x), you got 16x-15
a(ax+b)+b=16x-15
distribute
xa^2+ab+b=16x-15d
we look at the x terms
therefor
xa^2=16x
a^2=16
a=-4 or 4
we will find out
look a constant terms
ab+b=-15
undistribute b
b(a+1)=-15
if a is -4 then
b(-4+1)=-15
b(-3)=-15
b=5
if a=4 then
b(4+1)=-15
b(5)=-15
b=-3
so
g(x)=-4x+5 or
g(x)=4x-3
if a=-4, b=5
if a=4, b=-3